This application is the national phase under 35 U.S.C. xc2xa7371 of PCT International Application No. PCT/SG97/00057 which has an International filing date of Nov. 6, 1997 which designated the United States of America.
1. Field of the Invention
This invention relates to the area of manufacturing components and devices for micro-mechanical, micro-optical, micro-fluidic, micro-electronic, micro-acoustical, and micro-chemical applications. and utilises high energy light ions for micromachining. It can be applied either independently of, or in combination with, other techniques for micromachining.
2. Background of the Invention
The prior art which relates to the present invention is the use of X-rays for micromachining, commonly referred to by its German acronym LIGA(E. W. Becker, et.al., Microelectronic Engineering 4(1986)35-56; W. Ehrfeld and H. Lehr, Radiat. Phys. Chem. 45(1995)349-365), which in its most common embodiment comprises four main process steps:
i) In the first step a layer of positive polymer resist, usually polymethylmethacrylate (PMMA), which is typically several hundred microns thick and adhering to a metal substrate, is exposed in a deep X-ray lithography process using X-rays (usually from a synchrotron source) through a proximity X-ray mask. The use of a planar mask with defined areas of high and low transparency to X-rays is essential to this step.
ii) In the second step a suitable chemical developer is used to remove the exposed volume of the resist and expose selected areas of the underlying metal substrate. This chemical developer must be highly specific in completely removing the well exposed regions of resist while leaving unexposed or marginally exposed resist unaffected (V. Ghica and W. Glashauser, Verfahren fuer die spannungsfreie Entwicklung von bestrahlten Polymethylmethacrylat-Schichten, Offenlegungsschrift DE 3039110 Siemens AG, Munich).
iii) The third step is to electroplate metal onto the exposed metal substrate until the deposited metal thickness is equal to the resist thickness. The remaining resist is then removed to leave metal structures protruding from the metal substrate.
iv) In a fourth step these metal structures may be used as a mould to form structures in other materials.
The merits of the LIGA process lie chiefly in the ability of the first two process steps to form microstructures of large structural height (tens of microns to a few mm), with aspect-ratios up to 100. The aspect ratio is defined as the ratio of the structural height to the smallest lateral dimension. In typical practice, polymer structures with lateral dimensions of several microns to several hundred microns are formed by process steps one and two as described above. The limitations of the LIGA process which are relevant to the present invention are associated with the first process step of deep X-ray lithography using a proximity X-ray mask, and consist of the following:
a) That without undue complexity such a process is only suitable for the production of prismatic polymer structures on a planar base with walls perpendicular to the planar base.
b) That in applications where it is desirable that the deposition of energy by the exposure process should be of limited range or depth that the X-ray exposure process is unsuitable. This is due to the fact that X-rays are attenuated by matter, but do not have a fixed or well defined range.
c) That the cost and effort involved in fabricating a mask for the X-ray exposure step is high for the fabrication of prototype and low volume of production microstructures.
d) That the adhesion of the resist structures to the substrate can be adversely effected by the undesirable exposure of the resist due to photoelectrons, auger electrons, and fluorescence x-rays emitted from the substrate following absorption of x-rays from the primary source (F. J. Pantenburg, et.al., Microelectronic Engineering 23(1994)223-226; F. J. Pantenburg and J. Mohr, Nucl. Instrum. and Meth. B97(1995)551-556)
It is a primary object of this invention to provide a means of exposing a resist, for the purpose of micromachining, by using a direct-write beam of energetic ions. Specifically, the type of ions employed in this invention are the isotopes of hydrogen, helium or lithium, with kinetic energies in excess of about 250 KeV.
Another object of this invention is to provide a means of creating microstructures in a resist many microns thick (eg greater than about 2 microns), which can either be of practical use in themselves, or to form microstructures in other materials, for example by electroplating onto a metal substrate.
It is a further object of the present invention that the microstructures created in a resist several microns thick (eg in the range 2 to 20 microns) can be of high-aspect-ratio (i.e. the height of the microstructures is large in comparison to their lateral dimensions).
It is another object of this invention to provide a means of exposing resist which overcomes many of the limitations associated with prior art, namely the deep X-ray lithography process which is the first step in the LIGA process.
Specific advantages of the present invention over prior art include, but are not limited to, a) the greater geometrical freedom in the microstructures which can be machined, b) reduction or elimination of damage to material underlying the resist, c) the ability to machine structures with sub-micron dimensions in resists of many microns thickness and d) the ability to micromachine structures without requiring a mask. Other objects, features and advantages of the present invention will become apparent from the detailed description which follows, or may be learned by practice of the invention.